Internal support function returned an error sending to Distribution list

Issue:  Customer reported “Internal support function returned an error sending to Distribution list”

Resolution:

There is a duplicate, one-off contact in the DL, containing the Internet address for a member of the Microsoft Exchange Server Global Address List (GAL).

Do not use the identical display name for one-off contacts in a DL that also have Internet entries in the GAL.

Customer is advised to re-create the distribution list in Outlook